1. Field of the Invention
The present invention relates to a self-acting air bearing and process of producing the same, and more particularly to a self-acting air bearing and process of producing the same employed in a laser scanner of a laser printer, a digital copying machine, a laser facsimile machine, etc.
2. Discussion of the Background
Generally, though a small size rotor is supported by an antifriction bearing, if the rotating speed becomes 10,000 rpm or more, problems of vibration, noise and life of a lubricating oil occur. Therefore an air bearing is often employed as a bearing at high speeds of 10,000 rpm or more for high accuracy.
As a self-acting air bearing which generates a pressure for supporting its self-weight by rotation of a rotor, a cylindrical bearing, Tilting-Pad bearing or grooved-journal air bearing is conventionally employed.
A cylindrical bearing is easy to produce or work. When the cylindrical bearing supports a horizontal rotor, an eccentric force is acted by its self-weight. Therefore if a bearing spacing is narrower, since the cylindrical bearing can support stably up to approximately 15,000 rpm, it was employed for supporting a rotor in a broadcasting VTR in some cases. However when it supports a vertical rotor, as whirl instability occurs from low speed, it cannot be employed.
Further, in a Tilting-Pad bearing, a bearing surface is divided into three or four arcs of pads and the pads are softly supported whereby stability of a rotor at high speed is provided. Therefore it is employed in a turbo expansion apparatus for liquid helium. However, since there are problems in that the accuracy of the bearing is low and producing the bearing is complicated, it is not suitable as an air bearing for consumer use or office use which requires high accuracy rotation and low cost.
There is a grooved-journal air bearing for solving the above problems, such as one with herringbone spiral grooves. In this bearing, spiral grooves are formed for involving an air from outside by rotation. Even if the bearing does not have eccentricity, a bearing reaction force is generated in a radius direction by these spiral grooves. Therefore even in a vertical rotor, the bearing can support stably up to high speed which is ten thousand rpm or more of revolution. In this case, etching, rolling or laser working etc. is employed for forming those grooves.
Moreover, a multilobe bearing which is stable up to high speed is employed as liquid bearing in a turbo apparatus, which is not employed as the air bearing. In the multilobe bearing, the cylindrical bearing is divided into two or three arcs and each arc is set closer than the position in cylindrical bearing. A large eccentricity is provided to the bearing in each arc portion thereby the whirl stability can be increased. However, as the bearing is divided, it is not easy to produce or work with high accuracy of .mu.m or less and an area where air pressure generated is small. Therefore it is not employed in an air bearing which requires minute bearing spacing of .mu.m.
From the above reasons, the grooved-journal bearing is often employed as a self-acting air bearing in the laser scanner wherein a vertical rotor is often employed.
However, in the conventional grooved-journal bearing, as the groove is formed by etching, rolling or laser working, etc., manhours for producing or working the bearing is increased and therefore a problem of high production cost occurs. Further, in the conventional grooved-journal bearing, though the rotation is stable, there is a problem that dynamic rigidity with respect to an unbalance vibration of a rotor is lower than in the cylindrical bearing.